The present invention relates to a line pressure control system for an automatic transmission, and more particularly to a system for controlling line pressure when the automatic transmission is shifting between two gear ratios.
In the automatic transmissions, a desired gear ratio is established by hydraulically activating selected one or ones of various friction devices (such as clutches and brakes) by line pressure, and a shift between two gear ratios is effected by changing friction device or devices to be activated.
If the line pressure is excessively high, transient engagement capacity of a friction device becomes excessively high, causing great shock to occur, whereas if the line pressure is excessively low, the transient engagement capacity of the friction device becomes excessively low, causing a slip to occur, thus shortening an operating life of the friction device. Thus, the line pressure has to be appropriately controlled. Conventionally, as described in the publication entitled "SERVICE MANUAL FOR AUTOMATIC TRANSMISSION OF THE RE4R01A TYPE" (A261C07) published by in March 1987 by NISSAN MOTOR COMPANY, LIMITED, a drain circuit of a line pressure regulator valve is opened or closed by a line pressure solenoid of the duty cycle type. The duty varies from 0% to 100%. When the duty is 0%, the line pressure solenoid is left OFF, closing the drain circuit, while when the duty is 100%, the line pressure solenoid is left ON, opening the drain circuit. Thus, increasing the duty causes the line pressure regulator to increase a magnitude of a line pressure generated thereby. Various values of the duty are contained in a table data in a ROM of a microcomputer of an automatic transmission control unit. Different table data are provided, one for use at shifting operation, another for normal stable non-shifting operation, for example. The duty values are arranged in each table data as being retrievable by table look-up operation using a variable such as a throttle opening degree.
However, this conventional line pressure control system cannot cope with a situation where the line pressure solenoid has manufacturing variation or the characteristic of the line pressure solenoid degrades with time or a situation where the friction device has a manufacturing variation or the frictional materal of the friction device degrades with time. In the former situation, even if the line pressure solenoid is subject to the same duty, the magnitude of line pressure deviates from a target value. In the latter situation, even if the magnitude of line pressure is adjusted to the target value, the friction device does not show a desired performance characteristic. Thus, in any event, the conventional line pressure control system fails to avoid occurrence of substantial shift shock or reduction in operating life of the friction device.
An object of the present invention is to provide a line pressure control system for an automatic transmission wherein a magnitude of line pressure is adjusted always to such an appropriate value as to cope with the above-mentioned situations.